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To mitigate climate change, it is of vital importance to increase the stocks of global soil organic carbon (SOC), which also improves soil resilience, soil fertility and thus food security. The preservation of SOC heavily depends on its vulnerability to microbial degradation. Two processes and their interplay strongly influence carbon protection: the formation of primary organo-mineral (O-M) complexes via the sorption of dissolved organic carbon (DOC) to fine-grained soil minerals, and the aggregation of these to form micro and macroaggregates. To date, research suggests that the chemistry of the SOC and the mineralogy of the soil matrix play a key role in the formation of O-M complexes and their stability against microbial degradation, but whether and to what extent these factors help control micro and macroaggregation are unknown. We focus our investigation on how the chemistry of the SOC source affects the stability and aggregation of iron (oxyhydr)oxide O-M complexes. Thus, we determine the sorption behaviour of different SOC sources chosen to represent different functional group chemistries, using sorption isotherm experiments and electrophoretic techniques. We also conduct long-term aggregation experiments to track aggregate particle size using a novel Particle Size and Shape Analyzer technique. Our findings suggest that the stability and aggregation modes of O-M complexes are a function of SOC chemistry, and that aggregation patterns are strongly influenced by the presence of microbial exudates and communities.

Phosphorus (P) is an essential element for biomass growth and is a major component of fertilizers applied to crops. Excessive application of P to agricultural soils may lead to P-leaching and increasing concentrations of this element in aqueous system. This may cause the degradation of water quality through eutrophication processes. Arsenic on the other hand is a very toxic element that may be present at high concentrations in soil and aqueous environments due to weathering processes or to anthropogenic sources as mining and agricultural activities.To design adequate remediation techniques for the mitigation of phosphorus (P) and arsenic (As) pollution it is key to understand the geochemical processes and the environmental drivers controlling their availability. In soil systems, the mobility of P and As is controlled by their interaction with soil minerals. Among these minerals, iron and aluminum oxyhydroxides are known to form especially strong surface complexes with both components, limiting its availability to the environment. On the other hand, the presence of soil organic matter may compete with these pollutants for the mineral surface sites, increasing the contamination risk in the soil system.In the present work, we have studied at laboratory scale the As and P adsorption process on iron mineral surfaces under variable conditions. This includes changes of pH, redox conditions, presence of major ions (i.e. calcium, sulfate) or other trace elements (i.e. chromate, copper), and the presence of natural organic matter (i.e. humic substances or simple organic acids). Also, surface complexation models were applied in order to simulate and predict the behavior of such systems. The obtained results reveal that the presence of organic matter or other anionic species are important factors capable of increasing the mobility of P and As contaminants, whereas changing the redox potential or the carbon content in the organo-mineral aggregates has little effect.The information obtained, allowed to design remediation solutions that could be applied at field scale. The soil characteristics and the biogeochemical processes can be improved by the application of amendments based on circular economy aspects, i.e. compost, biochar, Technosols. We have studied the effectiveness of the superficial application of these materials to immobilize both P and As, varying the main environmental drivers affecting to the sorption or desorption processes.

The potential of composts produced from different origin residues to be used in environmentally friendly agriculture is addressed in this work. Seven composts obtained from different raw materials and composting methodologies are compared using elemental, thermal and spectroscopic characterization data. Despite the stabilization of the organic matter in all composts being adequate for agricultural applications, they display distinct elemental and structural compositions. Likewise, the fertilisers have very different effects on lettuce growth. Despite the observed differences, some common features were found, namely a mass loss (TGA) of 25.2 g per mol C, association between groups of elements (Fe, Al, Ni, Co, Cr, Cu and S; Mg, Na, K and P, C, Coxi, N and Pb) and correlations between the amount of carbon nanostructures and the characteristic aromaticity parameters. These results suggest that the tuning of the compost features for specific cultures may be possible for sustainable food production., This work was financially supported by the Interreg VA Spain-Portugal Programme (EU) through the project Res2ValHum (0366_RES2VALHUM_1_P). A.C. Silva acknowledges receipt of a PhD grant (UMINHO/BD/40/2016) financed by the Operational Programme Norte 2020 (through the Project “NORTE-08-5369-FSE-000033”). A. Teixeira acknowledges the grant (Res2ValHum 01/2018) to develop experimental work for 11 months on the project. J. Antelo and S. Fiol are also grateful for the financial support provided by Xunta de Galicia—Consellería de Educación e Ordenación Universitaria de Galicia (Consolidation of Competitive Groups of Investigation; GI-1245, ED431C 2018/12 and CRETUS AGRUP2015/02, ref. 2018-PG10).

The Olsen method is widely used to determine bioavailable phosphate (P) in upland soils. It is also used in flooded soils, although different estimates of extractable-P are obtained under anoxic and oxic conditions. In this study, variations in extractable-P in three soils under different redox conditions were evaluated as a function of solid to solution ratio (SSR) (1:5-1:200) and bicarbonate concentration (0.1-1 M). The parameterized CD-MUSIC model was used to describe the data, with optimization of reactive surface area (RSA) and reversibly adsorbed-P (R-PO

The potential of a calcium silicate hydrate waste product to remove phosphate from sewage treatment plants was tested by using the residue as the sorbent material in an experimental filter bed. Pseudo-second order and intraparticle diffusion models were used to explain the kinetics of phosphate adsorption on the solid material, and the Freundlich equation successfully predicted phosphate adsorption under equilibrium conditions. The results of the sorption experiments demonstrated that the retention of phosphate is mainly controlled by adsorption processes, although a small fraction is removed from solution via precipitation. The possible reutilization of the phosphate-saturated residue to amend phosphorus-deficient soils was also evaluated. The findings confirm that the phosphate saturated waste can be included in Technosols and used to amend phosphorus deficient soils.

Schwertmannite is a metastable mineral playing a crucial role in the immobilization of metal(oid)s in acid mine drainage (AMD) systems. High temperatures associated with wildfires could lead to a sudden schwertmannite transformation, changing the mobility of metal(oid)s. The objective of the present study was to examine the thermal transformation from schwertmannite to hematite, and the subsequent effect on the chromium partitioning. The immobilization of arsenate after thermal transformation and its implications on chromium mobility was also evaluated. Natural schwertmannite, with increasing contents of chromium, was thermally treated between 200 to 800 °C. Transformation products were characterized by solid-phase techniques and selective chemical extractions. Results indicated a transformation to hematite at temperatures above 400 °C. The presence of chromium barely affected the temperature at which the transformation occurred, although partitioning of chromium in the mineral changed with temperature. As the temperature increased from 25 °C to 400 °C, chromium was less mobile and less outcompeted by arsenic adsorption, suggesting a larger contribution of inner-sphere complexes with increasing temperature. At temperatures above 600 °C, non-mobile forms strongly associated with neo-formed hematite were found. Finally, neo-formation of hematite led to a decrease in arsenic adsorption, implying a potentially enhanced arsenic mobility in AMD systems upon wildfires.

So far, research on the microalga

So far, research on the microalga Haematococcus pluvialis has been focused mainly on the exploitation of its high astaxanthin content, leaving aside the use of other bioactive compounds present. This study is focused on obtaining and characterizing extracts enriched in bioactive compounds from this microalga red aplanospores. This is performed by means of Matrix Solid-Phase Dispersion (MSPD) extraction process, in an environmentally friendly way with low energy consumption and GRAS solvents. The effects of extraction parameters, particularly the extraction solvents (ethanol, ethyl lactate and water) are studied, in order to obtain maximum recovery of the main antioxidant compounds of interest (carotenoids, fatty acids and derivatives). Characterization of extracts is carried out by HPLC-DAD (High Performance Liquid Chromatography Diode Array Detector) and UHPLC-QToF (Ultra High-Performance Liquid Chromatography Quadrupole Time-of-Flight). The results show that MSPD produced extracts with higher bioactive compound recoveries than conventional cell disruption extractions. At the same time, a novel untargeted characterization for this species is performed, identifying compounds not previously dated in H. pluvialis, which include 10-phenyldecanoic acid and the -oxo and -hydroxy derivatives of palmitic acid. This approach, first applied to a freshwater microalgae, characterized by rigid and resistant aplanospores, provided a synergistic and sustainable extract, giving a broader focus on the use of this microalga This work was supported by project EQC2018-005011-P (Ministry of Science, Innovation and Universities, Spain). The authors belongs to CRETUS Institute and CRETUS Strategic Partnership (ED431E 2018/01) co-funded by FEDER (UE) SI

Phosphorus is an essential element that at high concentrations generates eutrophication of aquatic systems. In this study, we used batch and continuous tests to evaluate the efficiency of a Technosol to retain the phosphorus present (as phosphate) in aqueous samples. Phosphate sorption on Technosol was studied through batch and continuous experiments. Sorption kinetics and isotherms were investigated at different phosphate loadings and pH. In batch tests, we have determined how the presence of different anions (bicarbonate, sulphate, chloride, chromate and molybdate) affected phosphate retention. In continuous flow systems, phosphate immobilization was assessed as a function of flow rate, pH and Technosol concentration. Finally, the potential reutilization of the column was evaluated using consecutive sorption-desorption cycles. Phosphate sorption follows a pseudo-second-order kinetics model and a Langmuir isotherm model. The maximum sorption capacity ranged from 7.1 to 18.5 mg g−1, with larger values obtained at the highest pH. The main mechanisms involved in the sorption process were precipitation (as Ca-P minerals) and surface adsorption. In the column experiments, we observed a sorption reduction from 6.19 ± 0.06 to 2.37 ± 0.06 mg g−1 as the flow rate increased from 1.5 to 5.0 mL min−1. In addition, the retention capacity decreased by 14% when the height of the reactive layer was halved. Finally, the retention capacity of the Technosol spolic silandic recovered well after several sorption-desorption cycles, reaching 40% of the original value after first and second cycles. The material effectively retained phosphate in batch and continuous flow systems. The Technosol spolic silandic is considered an efficient sorbent to remove the excess of phosphate from the soil solution and the aqueous system. This material may be a useful tool to mitigate or minimize two important environmental problems: eutrophication and the scarcity of natural sources of phosphate. The Technosol can thus be recycled as a phosphate-rich amendment and the leachates can be used to produce liquid fertilizer.

Organo-mineral composites formed by the association of iron (hydr)oxides and organic matter are widespread in natural environments and play an important role as scavengers of bioessential elements and contaminants. To better understand the mobility and fate of Cu in natural soils and sediments we precipitated ferrihydrite and goethite organo-mineral composites using humic acid as an analogue for natural organic matter, with organic carbon content in the composites ranging from 2 to 16 wt% C. We then measured the adsorption of Cu to the end-member mineral and organic phases and the composites as a function of pH and Cu concentration. We determined the molecular mechanisms of Cu adsorption to the end-member phases and the composites, and used this information to develop molecularly constrained thermodynamic surface complexation models to quantify Cu adsorption. By combining our work here with previous work on the adsorption of Cu to ferrihydrite-bacteria composites, we provide insight into the predominance of Cu-carboxyl binding for Cu adsorption to iron (hydr)oxide organo-mineral composites, and the nature of Cu adsorption behaviour across a range of iron (hydr)oxide composites composed of different minerals and different types of organic matter. Taken as a whole our results show that Cu adsorption to the carboxyl group present in organic matter coatings on iron (hydr)oxides is likely common to most iron (hydr)oxide composites, such that Cu-carboxyl binding provides a key control on the fate and mobility of Cu in soils and sediments. Our work also suggests there is a universal adsorption behaviour for Cu adsorption to ferrihydrite organo-mineral composites, in which the mineral:organic mass ratio is a crucial parameter for determining Cu uptake. Overall we show that ferrihydrite composites composed of different types of organic matter and containing a wide range of organic mass ratios, but where the mineral is the dominant composite fraction, possess additive Cu adsorption behaviour which can be predicted using a component additivity surface complexation model.

Secondary iron minerals are important in mining areas as they are natural scavengers of many contaminants. Thus, aluminium and iron oxides derived from acid mine drainage determine the fate of trace metals commonly found in mining areas. They mainly do this via sorption processes, which prevent leaching of the metals and contamination of surface and groundwater. In the present study, abiotic precipitates were obtained from samples of acid drainage water collected from an abandoned copper mine. Characterization of the precipitates revealed that schwertmannite was the dominant iron mineral phase present. Retention of Cu, Cd, Ni and Pb by natural iron precipitates was compared with retention of the same elements by synthetic analogues. The study findings indicate that the natural abiotic precipitates have a higher retention capacity than their synthetic analogues. Thermodynamic modelling was used to predict the geochemical behaviour of the trace metals in the presence of iron precipitates. Inclusion of adsorption and precipitation reactions in the models yielded a good description of the degree of attenuation of adverse effects of the trace metals. Under acidic conditions, the schwertmannite-like particles initially present in the natural precipitates evolve towards more crystalline mineral phases. This transformation process is accompanied by the release of sulphate ions into the solution and by a decrease in trace metal sequestration.

Composting and vermicomposting have attracted attention in relation to both waste management and the potential to produce organic amendments that could improve soil quality. The main differences between compost depend on the feedstock, the production process, and the degree of maturity. In the present study, samples of compost of different origin (food and green waste, livestock waste, algae waste, urban waste or sewage sludge) or subjected to different composting methods (traditional or using earthworms) were collected for analysis. Additionally, samples collected at various stages of the composting process were compared (raw material, 15 and 30 days of composting, and final compost). Different analysis and techniques were used to establish the chemical composition, physicochemical and acid-base properties of compost samples and the organic matter extracts. The correlations obtained (between the abundance of acid groups in different extracts of the compost or between the cation exchange capacity and the C/N atomic ratio) would allow for predicting the compost behaviour based on certain characteristics, and a reduction in the number of parameters determined experimentally, thus facilitating comparisons between different compost. In addition, the potential value of the compost as amendment was tested with a Haplic Cambisol from a mining area. The application of compost increased the pH, the organic matter and nutrient content, and promoted seed germination and root growth.

Goethite is a common iron oxide present in the environment. It is used as a model system to represent the mineral fraction of the soil, whereas humic acid is used as a model system to represent natural organic matter (NOM). In this study, we tried to understand the behaviour of anionic and cationic pollutants in a model system, which could easily represent the organic-mineral interaction in the environment. Specifically, the retention of arsenate and copper over different organo-mineral systems was studied. The obtained results show an increase in the adsorption of copper as the pH and the C content in the organo-mineral composites was raised, whereas in the case of the arsenate, the opposite behaviour was observed. The adsorption behaviour was compared with that of the end-members goethite and humic acid. Furthermore, the NOM-CD model (natural organic matter-charge distribution model) was used to describe the reactivity of the pollutants in the solid/solution interphase and to predict the surface and aqueous speciation. To simulate the behaviour of the organo-mineral composites, a surface component was included to describe the chemical and electrostatic interactions of the NOM with the goethite in the modelling of the solid/solution interphase. This approximation allows the quantification of the effective organic matter, which can be used to describe the copper and arsenate reactivity in organo-mineral systems.

Secondary iron oxides formed in acid mine drainage, such as schwertmannite, are scavengers for metal(loid)s in mining environments. Increasing the understanding of the geochemical transformations of these minerals, as well as knowing how metal(loid)s affect these transformations, is crucial to ultimately predict the fate of these trace elements in acidic mine drainage and to minimize the potential environmental risk. In this study, transformation experiments have been conducted with a schwertmannite-rich sediment collected from a mining area and with synthesized schwertmannite as a reference material. The transformation of schwertmannite into goethite was studied as a function of the presence of arsenic, pH value, and redox conditions. Arsenic delayed the mineral transformation from pseudo-stable amorphous phases to more stable crystalline forms, especially at higher arsenic loadings and more acidic pH. Experiments in the presence of Fe(II) and ascorbic acid have proven that both components promote the mineral transformation or reductive dissolution of schwertmannite under anoxic conditions. The presence of arsenic reduced the catalytic effect of Fe(II), stabilizing the schwertmannite particles. On the other hand, arsenic had no effect on the reductive dissolution at these conditions when ascorbic acid was used as a reducing agent.

Biochar samples were generated by low temperature pyrolysis of different types of waste. The physicochemical characteristics of the different types of biochar affected the copper retention capacity, by determining the main mechanism involved. The capacity of the biochar to retain copper present in solution depended on the size of the inorganic fraction and varied in the following order: rice biochar>chicken manure biochar>olive mill waste biochar>acacia biochar>eucalyptus biochar>corn cob biochar. The distribution of copper between the forms bound to solid biochar, dissolved organic matter and free organic matter in solution also depended on the starting material. However, the effect of pH on the adsorption capacity was independent of the nature of the starting material, and the copper retention of all types of biochar increased with pH.

Low redox potential in flooded soils may affect phosphate bioavailability by reducing iron oxides or formation of new minerals. To investigate phosphate behavior in anoxic conditions, goethite was selected as a soil model and coated by humic acid (HA) and sodium borohydride was used as a reducing agent. Adsorption experiments were conducted in 0.1 M NaNO3 as a function of pH in oxic (Eh = +254 to +448 mV) and suboxic (Eh = −162 to +167 mV) conditions for four phosphate concentrations (0.05–0.8 mM). CD-MUSIC and NOM-CD models in combination with Extended Stern model were used to describe the experimental data. Results show that by increasing pH and carbon content in the organo-mineral composites, the released phosphate to the solution increases in both oxic and suboxic conditions. In suboxic conditions, as a result of sodium borohydride dissociation in water and consequently boron release to the solution, at high loading of boron and low loading of phosphate, boron can compete with phosphate for the surface reactive sites and decrease its adsorption. On the other hand, ferrous iron can attenuate boron effect and promote phosphate adsorption. The results indicated that goethite surface is resistant to the reductive transformation that may occur at relatively low redox potential due to its high crystalline character and thermodynamic stability. HA may, however, promote the formation of amorphous iron phases, which consequently might induce phosphate adsorption in OM-mineral composites. The derived affinity constants in oxic conditions described the experimental data of suboxic conditions reasonably well.

Pyrolysis of waste materials to produce biochar is an excellent and suitable alternative supporting a circular bio-based economy. One of the properties attributed to biochar is the capacity for sorbing organic contaminants, which is determined by its composition and physicochemical characteristics. In this study, the capacity of waste-derived biochar to retain volatile fuel organic compounds (benzene, toluene, ethylbenzene and xylene (BTEX) and fuel oxygenates (FO)) from artificially contaminated water was assessed using batch-based sorption experiments. Additionally, the sorption isotherms were established. The results showed significant differences between BTEX and FO sorption on biochar, being the most hydrophobic and non-polar contaminants those showing the highest retention. Furthermore, the sorption process reflected a multilayer behaviour and a relatively high sorption capacity of the biochar materials. Langmuir and Freundlich models were adequate to describe the experimental results and to detect general differences in the sorption behaviour of volatile fuel organic compounds. It was also observed that the feedstock material and biochar pyrolysis conditions had a significant influence in the sorption process. The highest sorption capacity was found in biochars produced at high temperature (> 400 °C) and thus rich in aromatic C, such as eucalyptus and corn cob biochars. Overall, waste-derived biochar offers a viable alternative to be used in the remediation of volatile fuel organic compounds from water due to its high sorption capacity.

The geochemical behaviour of phosphate and arsenate ions in soil and aquatic systems is determined by the presence of mineral surfaces and major ions. Information about the distribution of oxyanions over the solid and solution phases is essential for understanding the transport, bioavailability and toxicity of these compounds in the environment. Here, we studied the adsorption of both arsenate and phosphate on ferrihydrite nanoparticles in the presence of calcium ions. The presence of calcium ions enhanced the retention of these oxyanions on ferrihydrite and vice versa. The arsenate–calcium and phosphate–calcium multi-component systems were described using a mechanistic surface complexation model. Use of this type of model enables prediction of the solution and surface speciation, along with analysis of oxyanion mobility in relation to environmental conditions. We were able to calibrate the charge distribution model with the macroscopic data obtained for the single-component systems, thus obtaining surface complexation constants for later use to simulate multi-component systems. The mutual interactions between arsenate and calcium were successfully described with these parameters, indicating that changes in the electrostatic forces at the solid/solution interface caused the observed enhanced adsorption. However, adsorption in the phosphate–calcium system was underestimated with the parameters obtained for the single-component systems, indicating that additional mechanisms or processes should be considered. Formation of insoluble mineral phases was ruled out, but the inclusion of a phosphate–calcium ternary surface complex improved the modelling predictions.

Acid mine drainage (AMD) constitutes a serious environmental problem in mining areas due to the acidification of soils and aquatic systems, and the release of toxic metals. Many of the pollutants that occur in AMD display a high affinity for the surfaces of the aluminium and iron oxides that are typically present in systems affected by AMD. This binding affinity reduces the mobility of trace metals and metalloids, such as copper and arsenic, thus helping to mitigate contamination of aquatic systems. In the present study, water samples and iron-rich bed sediments were collected in areas affected by copper mining activities. A loose ochre-coloured precipitate occurring on the banks of a river close to an abandoned tungsten and tin mine was also sampled. The composition of the precipitate was established, and adsorption experiments were performed with copper and arsenate ions to determine the ability of natural iron precipitates to reduce the concentration of these ions in solution. Surface complexation models provided a good description of the behaviour of natural iron oxides in terms of copper and arsenate retention. Use of this type of model enables prediction of the distribution of pollutants between the solid and solution phases and analysis of their mobility in relation to environmental conditions (pH, ionic strength, presence of competing species, etc.).

Surface complexation models have proved to be valuable tools for predicting processes that occur at the solid-solution interface. Use of such models has become more widespread and nowadays more complex systems are studied, in an attempt to explain processes such as the competition between different species for mineral surfaces and the effect of the presence of organic matter. The aim of the present study was to analyze the mobility of phosphate in ferralic soils. The charge distribution model parameters for phosphate-goethite adsorption were used to predict phosphate mobility on samples from 2 horizons of a ferralic soil containing large amounts of iron oxides. The soil reactivity was attributed to the iron oxides, and some specific parameters were determined by means of phosphate adsorption-desorption experiments and included in the model. Adsorption of phosphate in the upper horizon, which contained more organic carbon and phosphate than the deeper one, was modeled by using the information obtained for the soil and the charge distribution model parameters derived for phosphate-goethite interaction with no need of further optimization. In contrast, some extra fitting parameters were required to improve the modeling of the phosphate adsorption in the deeper horizon.

High levels of oxyanions are found in the soil environment, often as a result of human activity. At high concentrations, oxyanions can be harmful to both humans and wildlife. Information about the interactions between oxyanions and natural samples is essential for understanding the bioavailability, toxicity, and transport of these compounds in the environment. In the present study, the authors investigated the reactivity of different oxyanions (AsO4 , MoO4 , SeO4 , and CrO4 ) at different pH values in 2 horizons of a ferralic soil. By combining available microscopic data on iron oxides with the macroscopic data obtained, the authors were able to use the charge distribution model to accurately describe the adsorption of these 4 oxyanions and thus to determine the surface speciation. The charge distribution model was previously calibrated and evaluated using phosphate adsorption/desorption data. The adsorption behavior on ferralic soil is controlled mainly by the natural iron oxides present, and it is qualitatively analogous to that exhibited by synthetic iron oxides. The highest adsorption was found for arsenate ions, whereas the lowest was found for selenate, with chromate and molybdate ions showing an intermediate behavior.

The formation, transformation and surface chemistry of iron oxides is geologically important in surface waters polluted by acid mine drainage (AMD). The geochemical behaviour of these oxides controls the availability and mobility of contaminants in such highly polluted systems. The low pH values, together with the presence of large amounts of sulphate, favour the formation of schwertmannite, which is a metastable iron oxide that is transformed to goethite under oxic conditions. However, in copper mining environments, co-precipitation of iron and copper ions is expected and therefore the surface chemistry of the iron oxides present in these systems may be different. Several schwertmannite samples were prepared in the presence of high concentrations of sulphate and different concentrations of copper to simulate copper-rich mining environments. Long-term transformation experiments were conducted to study the mineral oxide stability and variations in surface chemistry produced over a period of 15 months. Schwertmannite-like particles were initially formed, but sulphate release and the dissolution and re-precipitation of iron and copper ions occurred under more acidic conditions throughout the experiment, which led to the formation of goethite-like particles. The capacity for arsenate adsorption was highest in the initial schwertmannite-like particles, and arsenate mobility increased in the presence of more stable iron mineral phases that were formed throughout the transformation experiment.

The presence of iron oxides may play an important role in controlling the mobility and availability of contaminants in soils and waters affected by acid mine drainage. The present study describes the uptake of arsenate, chromate and molybdate from solution by synthetic schwertmannite. Batch experiments were performed at different pH values in order to obtain the adsorption isotherms for the three oxyanions. In addition to the formation of surface complexes between the oxyanions and the iron surface reactive groups, it is also expected that anion exchange will occur between sulphate anions from the schwertmannite structure and the oxyanions present in the solid/solution interface. Comparison of the experimental adsorption results for the different oxyanions showed large differences, not only the amount adsorbed, which was much higher for arsenate, but also in the sulphate exchange with the anions in solution. In case of chromate, the main mechanism of adsorption process is the exchange reaction with the sulphate groups present in the schwertmannite. The observed results suggest a different adsorption mechanisms for each of the three oxyanions, with important implications for the mobility of these anions in acid mine drainage systems.

For accurate interpretation of cation binding to natural organic matter, the proton binding behaviour of both solid and dissolved natural organic matter must first be established. In the present study, potentiometric titrations of samples of humin and humic acid extracted from a peat soil were performed at different ionic strengths. Humic acid (HA) samples in solution (dissolved humic acid, DHA) and in suspension (aggregated humic acid, AHA) were titrated. The corresponding charge curves were analysed with the NICA-Donnan model and the results were compared with those previously obtained for the peat soil. Good reproduction of the DHA charge curves was obtained, and in the case of the AHA, the exact forms of the charge curves were not reproduced at pH < 6 because of the conformational changes and dissolution of the sample that took place throughout the titration. The peat and humin displayed similar proton binding behaviour, which was different to that of the humic acid. With the Donnan approach, the value of parameter b, that relates the Donnan volume to the ionic strength, was the same for peat and humin and less than that of the humic acid, for both the dissolved samples and those in suspension, indicating that the effect of ionic strength on the charge is greater in the peat and humin than in the HA. The ratio between the contents of phenolic groups and of carboxylic groups was greater in peat and humin than in humic acid. The model used revealed that the acid-base behaviour of the peat is closer to that of the humin than to that of the humic acid. © 2012 The Authors. Journal compilation © 2012 British Society of Soil Science.

A sample of an ombrotrophic peat was washed thoroughly with nitric acid to release bound cations, and potentiometric titrations were carried out with the untreated peat and the acid-washed peat at five different ionic strengths of between 0.01 and 0.3. M. The experimental titration curves were used to obtain the absolute charge-pH curves for the acid-washed peat and relative charge-pH curves for the untreated peat (i.e. for the latter only the surface charge due to ionization of acid groups, as the value of the initial charge was not known because of the presence of exchangeable cations). The effect of ionic strength on proton binding was the same for the acid-washed peat and the untreated peat, as indicated by the similar spacing between the charge curves for both types of samples. The charge curves for the acid-washed and untreated peat corresponded after correction of the charge of the untreated peat, considering that the initial charge is equal to the charge on the exchangeable cations. The NICA-Donnan model, in which the Donnan volume varied with ionic strength, provided an excellent fit to the experimental data for the acid-washed peat and enabled the values of the intrinsic proton binding parameters to be obtained. Good simulation of the experimental charge curves for the untreated peat was obtained from the parameters of the acid-washed peat, by use of the same values of the Donnan volume, and including the charge due to the exchangeable cations. The parameters of the acid-washed peat therefore provide a realistic representation of the observed behavior of the untreated peat, and enable prediction of the behavior of peat in the natural environment. © 2011 Elsevier B.V.

Potentiometric titrations of a soil extracted fulvic acid solutions were carried out varying the initial concentration and the background electrolyte (KNO x ) concentration in the range from 5 10 -2 M to 1,0 M, The mastercurve approach was tried, but the data did not merge into a single curve, so the CA and LOGA-1 methods were applied to the experimental O(pH) curves to analyse the ionic strength effect over the fuivic acid ionization.

Adsorption of cationic pesticides in soils is generally attributed to mineral clays and organic matter components. However, iron oxides may also contribute to such adsorption or affect it by associating with other components. Using goethite and humic acid as models for iron oxides and organic matter respectively, we studied the adsorption of the cationic pesticide paraquat on goethite and humic acid-coated goethite. At pH 4.0 the adsorption on goethite was not significant, and at pH 10.0, although the surface of the oxide was negatively charged, much less pesticide was adsorbed than on mineral clays. At this pH the adsorption of paraquat decreased as the ionic strength increased, and application of the charge distribution multisite complexation model (CD-MUSIC model) enabled interpretation of the results. At pH 4, the adsorption of paraquat on the humic acid-coated goethite was similar to the adsorption on mineral clays, but was considerably less than the adsorption on humic acid in solution. The lower adsorption on solid organic matter is attributed to a decrease in the number of "active" binding sites on the humic acid as a result of the binding to iron oxide.

Differential pulse polarography (DPP) was used to study the complexation of cadmium by cysteine (CySH) at pH = 7.8 and l = 0.7 M (KNO3), as well as to determine the stability constants of the equilibria Cd2+ + CyS− ⇄ Cd(CyS)+ and Cd(CyS)+ + CyS− ⇄ Cd(CyS)2. Copyright © 1993 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim

The effect of pH on the adsorption of copper (Cu), lead (Pb) and cadmium (Cd) by a peat soil was studied, and the results compared with those corresponding to cation binding by a dissolved peat humic acid (HA), and interpreted with a NICA-Donnan model. A potentiometric titration technique was used to determine the adsorption isotherms for H + , at different ionic strengths, and for Cu2 + , Pb 2+ and Cd 2+ at different pH values, in a peat soil. The effect of the ionic strength on proton binding was similar for the soil (solid) organic matter and for dissolved HA. The adsorption isotherms for cation-peat and the binding curves cation-dissolved HA are almost parallel, although more cation was adsorbed per kg of C in the dissolved HA. The effect of pH on cation binding is similar for dissolved organic matter and for the organic soil. At low metal concentration the amount of adsorbed metal followed the order Cu 2+ > Pb 2+ > Cd 2+ . The cation-binding parameters obtained with the NICA-Donnan model allow excellent simulation of the effect of pH on the adsorption of Cu, Pb and Cd ions in the studied peat soil. The binding constants for the peat suspension were greater than the corresponding generic parameters for dissolved HA. Speciation calculations showed that for Cu and Pb, the most abundant fraction was the metal adsorbed on peat, whereas for Cd the most abundant fraction was dissolved metal.

We studied the acid-base properties of 16 fulvic acids and 16 humic acids isolated from the surface (3-15 cm) and subsurface (> 45 cm) horizons of two types of acid forest soils, derived respectively from amphibolite and granite rocks, under five different types of vegetation. The observed differences between the contents of humic substances in the two types of soils were related to the degree of Al-saturation of the soil organic matter, as indicated by the molar ratio between pyrophosphate extractable Al and C. Humic fractions were characterized in terms of elemental composition, and CPMAS 13C NMR spectrometry. The contents of carboxylic and phenolic groups were estimated by potentiometric titrations conducted in 0.1 m KNO3 in a nitrogen atmosphere. The fulvic acids contained more carboxylic groups but less phenolic groups than the humic acids: the ratio of phenolic to carboxylic groups in the humic acids was 0.48 ± 0.10 and in the fulvic acids 0.23 ± 0.05. The mean values of the protonation constants of each of the humic substance fractions can be used as generic parameters for describing the proton binding properties. The fulvic acids isolated from the subsurface horizon of the soil contained between 2.6 and 23% more carboxylic groups, and the humic acids between 8 and 43% more carboxylic groups than those isolated from the surface horizon of the same soil. © 2008 The Authors.

We have tested to see if the generic set of NICA-Donnan model parameters, used to describe isolated humic substances, can also describe soil humic substances in situ. A potentiometric back-titration technique was used to determine the variable surface charge of two organic peat soils at three different ionic strengths. The non-ideal, competitive-adsorption NICA-Donnan model was used to simulate the surface charge, by assuming a bimodal distribution of H + affinity on the soil solid phase. The model provided an excellent fit to the experimental data. The Donnan volume, V D , varied slightly with ionic strength, although the variation was less than for humic substances in solution. The values obtained for the parameters that define the affinity distributions, the intrinsic proton binding constant (log K i int ) and the heterogeneity of the site (m i ), were similar to those observed for isolated soil humic acids. The abundance of carboxylic groups in the whole soil represented 30% of the typical value for isolated soil humic acids. The composition of the organic matter of the whole soils, obtained by 13 C CPMAS NMR, was comparable to the characteristic composition of soil humic acids.

We studied the binding of Pb(II), Cd(II), and Cu(II) to humic acid and fulvic acid extracted from an ombrotrophic peat bog, by metal titration experiments at pH 5.5 and 6.0, and 0.1 M KNO3 ionic strength. Free metal ion concentrations in solution were measured using ion selective electrodes. The amounts of base required to maintain constant pH conditions were used to calculate H+ / M2+ molar exchange ratios For all three metal ions the amount of cation bound to the humic acid was greater than the amount bound to the fulvic acid. The isotherms for Pb showed saturation, with a maximum adsorption capacity almost the same as that observed for Cu. In the case of Cd, the saturation was not reached in such a clear way. At the lowest concentrations of metal, the slope of the log-log plot of the cadmium binding isotherm was intermediate between that of copper (the lowest) and that of the lead (the highest). The proton to metal exchange ratios followed the order Cu2+ > Pb2+ > Cd2+. The binding data were quantitatively described with the NICA-Donnan model, which allowed characterization only of the carboxylic type binding sites. The values of the binding constants were higher for the humic acid than for the fulvic acid, and for Pb than for Cd. The values obtained for the exponent ni allowed analysis of the stoichiometry of the binding reactions. © 2005 Elsevier B.V. All rights reserved.

Peat humic and fulvic acids were isolated from a surface horizon (0–60 cm) and deeper horizon (>60 cm) of an ombrotrophic peat bog located in Galicia (NW Spain) and which constitutes a record of paleoenvironmental contamination. The humic and fulvic acids were characterized and compared using chemical methods and spectroscopic techniques, including ultraviolet visible (UV-vis), Fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (13C NMR). Comparison of the E4/E6 ratios showed that in the deeper horizon the peat humic substances have a greater degree of aromatic condensation than in the surface horizon. The alkyl C content of the humic substances remained practically unchanged, whereas the O–alkyl–C content of humic acids decreased with depth. No variation with depth in the ionization constants of the carboxylic groups or of the phenolic groups was observed. The values of the ratios of the carboxylic/phenolic contents were similar to those reported in the available literature, with a higher content of acid groups in the humic substances isolated from the lower horizon mainly attributable to the carboxylic groups.

We studied the effect of pH on the binding of Cu2+ to a soil fulvic acid in solutions containing dissolved organic carbon at concentrations within ranges typical of freshwaters and soil solutions. ...

The binding of Cu2+ and Ca2+ to a fulvic acid (FA), simulating naturally occurring conditions, was studied. Furthermore, copper-FA complexation in the presence and absence of calcium was compared. For this, potentiometric titrations were carried out using a solution of FA at a concentration of 100 mgL(-1), and of ionic strength 0.1M. The Ca(2+)-FA complexation reaction was carried out at pH 6.5, 7.5 and 8.5 and the Cu(2+)-FA complexation reaction at pH 5.5 and 6.5, in both the presence and absence of calcium ion. The calcium ion had a significant effect on copper binding at [Ca2+]> or =2.5x10(-3)M. The experimental binding curves were analyzed using non-electrostatic discrete site and continuous distribution models. Competition by the calcium ion mainly affected the maximum binding capacity of the Cu(2+)-FA complexation, whereas the slight effect observed on the binding constant appears to indicate that calcium only competes for specific copper sites, thus modifying the binding sites distribution function. It was also found that the number of sites occupied by the copper ion represented only 20% of the total concentration of acid groups ionized on FA at the pH values studied.

Potentiometric titrations of a sample of soil extracted fulvic acid (FA) were carried out using different concentrations (20 mg l−1

The acid-base properties of two fulvic acids (FA) extracted from senescent leaves of eucalyptus and oak were characterized by carrying out potentiometric titrations at two FA concentrations and four ionic strengths (0.005 M[KNO(3)]1.0 M). Experimental data were analyzed by means of the master curve approach, which includes an electrostatic spherical double layer model, and the Langmuir-Freundlich isotherm was used to fit the data. The contribution of the electrostatic effect to the proton binding reaction was lower than that observed for soil fulvic acids. The chemical heterogeneity of both samples was described by two acid sites with p Ks of about 4 and 7.5, the most abundant being the carboxylic site of p K = 4.

A simple electrostatic model was used to study the effect of pH on the binding of Cu2+ to fulvic acid in solutions containing similar amounts of dissolved organic carbon (DOC) as natural media, such as aquatic environments and soil solutions. Complexation behavior was affected by increased pH because of changes in the electrostatic interaction resulting from an increase in the negative charge on the fulvic acid molecule. Solutions of soil-extracted fulvic acid (FA), at concentrations of 25 and 35 mg L(-1), ionic strength 0.005 M, and pH 5.0, 5.5, 6.0, 6.5, 7.0, and 7.5, conditions that simulate those of natural freshwaters, were titrated with copper ion using differential pulse anodic stripping voltammetry. Assuming the formation of 1:1 complexes, the conditional binding parameters (stability constant and binding capacity) were calculated for each pH value. Use of a 1:1 electrostatic model allowed estimation of the contribution of the electrostatic effect to the ion binding reaction, at each pH value, as well as calculation of a binding constant that was not dependent on pH and which thus represented the contribution of the chemical heterogeneity. Furthermore, it was found that only a small proportion of the FA acid functional binding sites are involved in the formation of copper complexes.

Surface complexation models have proved to be valuable tools for predicting processes that occur at the solid-solution interface. Use of such models has become more widespread and nowadays more complex systems are studied, in an attempt to explain processes such as the competition between different species for mineral surfaces and the effect of the presence of organic matter. The aim of the present study was to analyze the mobility of phosphate in ferralic soils. The charge distribution model parameters for phosphate-goethite adsorption were used to predict phosphate mobility on samples from 2 horizons of a ferralic soil containing large amounts of iron oxides. The soil reactivity was attributed to the iron oxides, and some specific parameters were determined by means of phosphate adsorption-desorption experiments and included in the model. Adsorption of phosphate in the upper horizon, which contained more organic carbon and phosphate than the deeper one, was modeled by using the information obtained for the soil and the charge distribution model parameters derived for phosphate-goethite interaction with no need of further optimization. In contrast, some extra fitting parameters were required to improve the modeling of the phosphate adsorption in the deeper horizon.

Potentiometric titrations of two soil extracted fulvic acids (FA) were carried out at various FA concentrations ( 25 mg l −1 FA ] mg l −1 ) and ionic strengths ( 0.005 M KNO 3 ] M ). Experimental data have been analyzed by means of the master curve approach which includes an electrostatic spherical double layer model, and Langmuir–Freundlich isotherm was used for fitting the data. For one of the samples, the chemical heterogeneity of the fulvic acid has been described by means of a unique master curve for all the concentrations studied. For the other sample, two master curves were necessary to analyze the chemical heterogeneity: one for the lowest concentration and other for the higher concentrations; however, the calculated intrinsic proton binding parameters for both master curves are not quite different. The concentration of carboxylic sites obtained from the Ca-acetate titration method matched the maximum charge corresponding to the acid carboxylic sites of the Langmuir–Freundlich isotherm quite well.

In the present work, the Cu2+-soil fulvic acid intrinsic complexation parameters (stability constants and complexation capacities) have been derived, using the master curve approach. The experimental technique was the potentiometry with an ISE for Cu2+ ions. The charge–pCu data derived from the experimental titration curves were transformed into charge–surface pCu (pCu=–log[Cu2+]) using a spherical double layer model, in order to obtain the intrinsic complexation constants. For this transformation, geometric parameters of the fulvic acid are needed. Several models have been applied that try to differenciate between heterogeneity and electrostatic effect, two main features of humic substances.

Potentiometric titrations of three fulvic acids (FA) extracted from samples located at different horizons of a soil were carried out at various FA concentrations (40, 80 and 120 mg l −1 ) and ionic strengths (0.01, 0.1 and 1.0 M KNO 3 ). Experimental data have been analysed by means of the master curve approach which includes an electrostatic double layer model. Spherical and cylindrical double layer models fitted the data equally well. Heterogeneity analysis of the master curves showed in all cases that the affinity distribution had two peaks centred at log K int H ∼4 and log K int H ∼8–9 , respectively. A double Langmuir–Freundlich isotherm was used for fitting the data. The number of weak acid sites derived from the equivalence point of the experimental titration curves matched the maximum charge corresponding to the more acidic carboxylic sites of the double L–F isotherm quite well. Even though a decrease in log K int H was observed with increasing FA concentration, we can conclude that the FA samples from the three horizons exhibit an almost identical behaviour.

We examined the variation with ionic strength ( I , adjusted with KCl, KNO 3 , KBr, NaCl or NaClO 4 ) of the formal potential ( E const ) for glass electrodes exhibiting a Nernstian response (i.e. E cell = E const − s log [H + ]). For this purpose, we investigated the different factors included in the formal potential, so we obtained reported values for the liquid junction potential as a function of ionic strength and determined the logarithm of the activity coefficient for the proton in various saline media, using Pitzer equations.

The dependence of the ionic strength of stoichiometric protonation constants, obtained potentiometrically, of several amino acids has been analysed by the mean spherical approximation (MSA), a mathematically simple statistical mechanical treatment based on integral equations. Unlike the Debye–Huckel equation, which overestimates the effects of ion–ion interactions as the sizes of all ions are neglected, the MSA represents the electrolytes as a mixture of hard spheres immersed in a dielectric continuum which represents the solvent. This approach fits experimental data using concentration, electrical charge and diameter concepts and is an alternative to semiempirical models based on different ionic strengths with coefficients that defy theoretical interpretation. Analysis of data using both semiempirical models and a quasi-lattice model, not containing Debye–Huckel terms, is also included.

The effect of the ionic strength on the second acid-base equilibrium constant of various aminoacids was analysed by use of a quasi-lattice model for log γ± over the ionic strength range 0.1 -1.5 M. Such treatment provides an alternative to empirical or semi-empirical equations based on modified versions of Debye-Huckel limiting law obtained by including different ionic strength terms. An interpretation of the minimum of the curve in terms of the model employed is also given. © by R. Oldenbourg Verlag, Munchen 1997.